A method is provided to indicate improper fuel filling in a vehicle. An indicator is actuated if a proportion of alcohol in the fuel is outside a range of expected proportions. In some examples, the range corresponds to a non-flex fuel vehicle recommended range, or to a normal range for a low alcohol-content fuel, which may be between 0 and 15 percent alcohol by volume. The indicator may include a diagnostic code or a dash light. Further, the indicator may be actuated in combination with a mitigating action, which may involve adjustment of a fuel injector pulse width or an ignition spark timing, as examples.